This invention relates to a flat circular waveguide (the so-called radial line type) device suitable for use as broadcasting antennas and the like.
A variety of flat circular waveguide devices has heretofore been proposed, including those having such a coaxial cable input structure as shown in FIG. 1 (indicated generally by a) and those having such a waveguide tube input structure as depicted in FIG. 2 (indicated as a whole by b). These conventional flat circular waveguide devices are accompanied by the following drawbacks irrespective of their structures:
(1) Power fed to a wave-guiding space c is subjected to attenuation to a considerable extent while it travels from the power-feeding portion to the terminal, as indicated by a solid line in FIG. 3 (The solid line corresponds to a flat circular waveguide device having slots d. The power density characteristic changes stepwise due to radiation lose through the slots d) and as indicated by a dashed line in FIG. 3 (the dashed line corresponds to a flat circular waveguide device having no slots are provided). Accordingly, the radiation power becomes uneven and the antenna gain is hence lowered significantly.
(2) A terminal resistor e (which is usually used for the distribution line type) is arranged along the periphery of the wave-guiding space c. This manner of arrangement requires use of the terminal resistor e which is elongated as a whole to cause a cost-up. Moreover, the size of the terminal resistor e must vary depending on the volume and size of the wave-guiding space c. Therefore, it is indispensable to provide terminal resistors of various sizes.
Incidentally, FIGS. 1 and 2 also illustrate an upper wall f formed of a metallic plate having the slots d therethrough, a lower wall g formed of a metallic plate, an inner conductor h1 of a coaxial cable, an outer conductor h2 of the coaxial cable, a waveguide i, a conductor matching plate j and an opening k.